Inductive charging systems designed for electric vehicles (EV) employ a transmitter coil, which is placed on or embedded in the road surface, to inductively transmit the charging energy via the air gap between the road surface and the vehicle to a receiver coil mounted to the underfloor of the EV. Similar to a traditional transformer, an alternating current in the windings of the transmitter coil is used to produce the magnetic flux specified to transfer the charging energy. Due to the large air gap inherent to the technology, the leakage flux of this transformer is high. It is known that the power efficiency of the transmission can be high despite the high leakage flux if one or more resonant compensation capacitors are connected to the terminals of the receiver coil. The structure of a known inductive charging system is shown in FIG. 1(a), with transmitter coils 3, 5 and compensation capacitors 6. The meaning of the remaining reference numerals is explained in the context of the other figures. The capacitors 6 form a resonant circuit with the receiver coil 4 inductance. If the transmitter coil 3 carries an alternating current with a fundamental frequency, which corresponds to the resonant frequency of the resonant circuit on the receiver side, the power is transferred with high efficiency.
A rectifier 13 and passive filter components 16 can be connected to the terminals of the receiver circuit to produce a steady dc-voltage from which the battery 2 is charged. To control the current in the transmitter coil 3, a full-bridge inverter 9 supplied from a constant dc-bus 10 voltage can be used. Other topologies, such as the three-level neutral point clamped converter could also be possible, but are rarely used because the voltages of interest for EV battery charging are more often lower than what could be used to take full advantage of the lower constraints regarding the blocking voltage of three-level topologies. In order to reduce the reactive power specified from the inverter, another resonant capacitor 5 can be connected to the terminals of the transmitter coil 3. The capacitance value of this second capacitor can be chosen such that the specified reactive power is minimized at the receiver-side resonant frequency, for example such that the input impedance of the circuit including the two transmitter coils 3, 4, the resonant capacitors 5, 6, and the load 2 appears ohmic at the resonant frequency of the receiver circuit.
According to an exemplary embodiment of the present disclosure, additional filter elements are connected between the transmitter-side or receiver-side power converters to reduce the stray fields caused by the currents in the transmitter coils. The full-bridge inverter 9 of the transmitter 8 is usually switched close to the resonant frequency with a phase-shift of the bridge legs close to 180°. Experiments can be performed to determine the resonant frequency of the actual charging system before operation or, alternatively, the resonant frequency is estimated from real-time measurements during operation. This determination is advantageous because the resonant frequency can deviate from its anticipated value due to tolerances of the components, temperature drifts, or due to a misalignment of the receiver coil with respect to the transmitter coil. The switching frequency of the full-bridge inverter 9 can then be adjusted to the actual operating conditions using the measured or estimated resonant frequency.
For the battery charging, it is specified that the battery current can be controlled. The battery can be charged according to a current and voltage profile that is specified based on the limitation of the charging current and the voltage stress of the battery, and adapted to the state-of-charge of the battery. A common charging scheme is shown in FIG. 1(b). The charging profile can also be designed to include other aspects, such as the minimization of the energy consumption during a charging cycle, or the minimization of the time specified for the charging process. A dc-dc-converter 15 can be connected in series to the dc bus of the rectifier of the receiver. The dc-to-dc-converter 15 is then connected to the battery via a low-pass filter 16 in order to eliminate the switching frequency ripple of the charging current. The converter is used to control either the battery current or the voltage applied to the battery according to the charging profile. Accordingly, the power that has to be transmitted from the transmitter to the receiver coil 4 is not constant, but depends on the state-of-charge of the battery 2. Because the transfer characteristics of the resonant system can exhibit a certain load dependency, the full-bridge inverter 9 at the transmitter side should be able to adapt to the actual load conditions. Additionally, the transfer characteristics of the resonant system can change due to a misalignment of the receiver coil 4 with respect to the transmitter coil 3 (cf. FIG. 4, 5), due to component tolerances, or due to parameter drifts. Hence, the full-bridge inverter 9 should also be able to adapt to the actual transfer characteristics due to these uncertainties.
According to known implementations, both adaptations can be realized by an adjustment of the inverter switching frequency. For example, if the resonance frequency is increased due to a coil misalignment, the actual resonant point should be tracked and the switching frequency should be increased accordingly in order to maintain a high efficiency. If the output power is reduced, the switching frequency should be shifted into a frequency region where the input impedance of the resonant system as seen at the output terminals of the full-bridge inverter is inductive in order to maintain a constant voltage at the output of the resonant circuit. While the switching frequency ensures Zero-Voltage Switching (ZVS) of the power semiconductors of the full-bridge inverter and leads to low switching losses, it causes additional conduction losses in the resonant circuit. Due to the inductive input characteristic of the resonant circuit, an increasing amount of reactive power is drawn from the full-bridge inverter in this operating mode. This results in reactive current components in both coils that cause conduction losses additional to those caused by the specified active component of the current.
Moreover, the current that has to be switched off by the semiconductors of the full-bridge inverter 9 can also be increased due to the reactive current components, which depending on the employed type of semiconductor can also cause additional switching losses. At the power and voltage levels of interest for EV battery charging, the Insulated-Gate Bipolar Transistor (IGBT) is often the preferred choice for the active power semiconductors as it offers a cost advantage and a high reliability. However, as described in G. Ortiz, H. Uemura, D. Bortis, J. W Kolar and O. Apeldoorn, “Modeling of Soft-Switching Losses of IGBTs in High-Power High-Efficiency Dual-Active-Bridge DC/DC Converters,” in IEEE Trans. Electron Devices, vol. 60, no. 2, pp. 587-597, February 2013, the charge stored in the junction of the IGBTs can lead to high tail-currents which can cause significant switching losses despite the ZVS conditions. In periods of low output power, the efficiency of the power conversion can be significantly lowered by these effects.
To supply the dc-bus 10 at the input of the full-bridge inverter 9 at the transmitter-side, a mains rectifier with Power Factor Correction (PFC) with an Electromagnetic Interference (EMI) filter is commonly used. Inductive charging systems designed for the power level of interest for EV battery charging can be fed from the three-phase mains. The structure of such an inductive charging system is shown in FIG. 1(a). A suitable converter topology comprises a mains rectifier 11 with three bridge legs that are connected to the three phases of the mains 1. To control the output voltage of the mains rectifier 11 the bridge legs are realized with active power semiconductor switches, such as IGBTs with anti-parallel diodes, and an inductor connected to each of the three input terminals of the PFC rectifier. A number of alternative converter topologies exist, possibly with fewer semiconductor switches or only a single inductor in the dc-link. A PFC rectifier can produce a controlled dc-voltage at a level above a certain minimum value given by the peak value of the mains line-to-line voltage while maintaining sinusoidal input currents in all three phases. It is therefore referred to as a boost-type PFC rectifier.
Other converter topologies exist that are able to produce output voltages below a certain maximum value given by the peak value of the mains voltage. These are commonly termed buck-type PFC rectifiers. An example is described in T. Nussbaumer, M. Baumann, J. W. Kolar, “Comprehensive Design of a Three-Phase Three-Switch Buck-Type PWM Rectifier,” in IEEE Trans. Power Electronics, vol. 22, no. 2, pp. 551-562, March 2007. As third alternative, for example CH 698 918 presents a buck+boost-type PFC rectifier, which allows supplying a dc-link voltage above or below the limit given by the peak value of the mains voltage. Of course, the same functionality can also be achieved by a boost-type PFC rectifier with a cascaded buck-converter, resulting in a boost+buck-type PFC rectifier. There exists a trade-off between the buck+boost-type and the boost+buck-type PFC rectifier structure in terms of the achievable conversion efficiency and power density of the converter, which should be taken into account together with the application and its specifications.
A variable dc-bus 10 voltage of the mains rectifier stage 11 provides another method to adapt to the variable load conditions on the receiver side. In this case, a change of the switching frequency of the full-bridge inverter can be omitted for the control of the output voltage and the switching frequency can, therefore, be constantly maintained at the resonant frequency of the tank, which results in a purely active current flow. This can significantly reduce the conduction losses in the semiconductor devices, the resonant capacitors, and in the transmitter coils. However, the switching losses of the power semiconductors of the full-bridge inverter can still be high due to high tail-currents of the IGBTs. They can present a limiting factor in the system design and make a compact realization of the converter difficult due to high cooling specifications. Additionally, for example in B. Goeldi, S. Reichert, and J. Tritschler, “Design and Dimensioning of a Highly Efficient 22 kW Bidirectional Inductive Charger for E-Mobility,” in Proc. Int. Exhibition and Conf. for Power Electronics (PCIM Europe), 2013, pp. 1496-1503 the adjustable dc-bus voltage is produced by a dc-to-dc-converter connected in series to a PFC rectifier. While this structure offers a higher modularity, the system efficiency is reduced by the higher number of conversion stages. Additionally, the higher number of components for the cascaded system makes a compact realization of the system challenging.